Various active control systems have been proposed and/or implemented that have controlled the brakes, steering and/or suspension of a motor vehicle to better allow a driver of the motor vehicle to maintain control of the vehicle under varying circumstances and conditions. In general, these control systems have attempted to improve motor vehicle performance in various driving conditions by coordinating control of multiple vehicle subsystems. Typically, such control systems have utilized a reference model, a state estimator and a vehicle control unit, which has incorporated feedback control in conjunction with feedforward control.
Similarly, a number of active control systems have been proposed to reduce the likelihood of motor vehicle rollover. In general, the design of these systems has been based on roll state dynamics. Typically, yaw rate stability control systems have been designed with consideration for yaw-plane motion and have ignored roll motion. Additionally, rollover stability control systems have been designed for roll motion and have ignored yaw-plane motion. In general, brake-based control designers have experienced difficulty in developing a strategy that coordinates rollover and yaw stability.
A number of motor vehicles have included electronic stability control (ESC), which is a closed-loop stability control system that relies on antilock brake system (ABS) and traction control system (TCS) components. A typical ESC system incorporates sensors for determining vehicle states, as well as an electronic control unit (ECU) to modulate braking and traction forces responsive to signals provided by the sensors. Various ESC systems have included wheel speed sensors, a steering wheel angle sensor, yaw rate and lateral acceleration sensors and master cylinder pressure sensors.
In general, the steering wheel angle sensor has provided a steering wheel angle and a steering input rate. The wheel speed sensors have provided signals that the ECU uses to compute the speed of the wheels. Typically, the vehicle speed is derived from the rotational speeds of all wheels using a computational algorithm. The yaw rate sensor has usually been implemented as a gyroscopic sensor that monitors a rotation about a vertical axis of the motor vehicle. The lateral acceleration sensor has been positioned to measure the acceleration of the vehicle in the direction of the lateral axis of the vehicle, i.e., the side-to-side motion of the vehicle. In a typical ESC system, the ECU includes a microprocessor that processes and interprets the information from each of the sensors and then generates necessary activation commands to control brake pressure and engine torque.
The concept behind an ESC system is to provide an active safety system that helps a motor vehicle operator prevent skidding that can occur in various kinds of weather, on different types of roads and in situations where even expert drivers may struggle to maintain their vehicles on the roadway. The stabilizing effect provided by an ESC system is based on calculations performed by the microprocessor of the ECU, which evaluates signals provided from the various sensors. The microprocessor utilizes the information provided by the sensors to continuously compare the actual and desired movement of the vehicle and intervene if the vehicle shows a tendency to leave an intended travel path. The ESC stabilizing effect is achieved by automatically applying a differential brake force (i.e., a difference between the left side and right side longitudinal braking forces), which affects the turning motion of the vehicle and helps to keep it on the intended path.
Typically, a control algorithm implemented by the microprocessor utilizes program setpoints, which are tailored to a particular vehicle and specific operations of the vehicle. The microprocessor of the ESC system then transmits appropriate commands to the braking system, to cause the braking system to provide a defined brake pressure at an appropriate wheel, depending upon the deviation of the motor vehicle from a desired path. The microprocessor may also command the vehicle to reduce engine torque during understeering or when wheel spin is detected during acceleration.
In this manner, an ESC system attempts to control the yaw rate and the slip angle of an associated vehicle. In general, the slip angle of a motor vehicle is difficult to measure, even with expensive sensors, and, as such, slip angle has usually been estimated. Typically, the slip angle has been estimated by using some type of observer model or by integrating a side slip rate. Unfortunately, current integration techniques have not properly accounted for sensor bias, sensor noise and/or banked road surfaces. While observer models are less susceptible to sensor related bias and noise, observer models may experience difficulty with banked roads, due to, for example, difficulty in accurately estimating tire forces.
What is needed is an accurate technique for estimating motor vehicle slip angle that accounts for banked roads.